This invention relates to inrush current limiters for incandescent lamp filaments and particularly to polycrystalline vanadium dioxide thermistors.
It has long been known that the large inrush currents typical of incandescent lamp filaments are detrimental and many suggestions of negative temperature coefficient thermistors have been made. The large inrush currents have detrimental effects both on the electrical systems supplying the power and on the lamps themselves. A number of detrimental effects on the lamp itself are known. These effects include, for example, the electromechanical effects of the large currents, but the most straightforward effect is probably the failure of the filament due to localized melting caused by local thermal overshoots which are in turn the result of the large inrush current. In this failure mode, a localized high resistance section is much more intensely heated due to its initial higher resistance. This effect is compounded as the positive temperature coefficient of the filament and the local heating combine to raise the resistance of the localized section more rapidly than the rest of the filament which results in even greater heating in the local section. Because the localized section heated up to its operating temperature much faster than the remainder of the filament and the total filament resistance has not risen to the operating value, the local section is subjected to higher than steady state operating currents even after the local section resistance has risen to or above its steady state operating value. This combination produces very intense local heating and temperatures even higher than the steady state operating temperature of that section. If a localized section reaches the melting point of the filament, the filament usually fails.
Despite the long-felt need, the thermistor configurations of the prior art have either failed to eliminate the inrush current overshoot or were excessively inefficient. Often the devices were both inefficient and ineffective. Several of the prior art devices, especially those with a very large thermistor mass or with a switching characteristic (as opposed to a more gradual change in resistance), delayed the current overshoot but did not eliminate it. While some of the configurations did extend the life of a lamp, this was primarily due to an excessive steady-state resistance in series with the filament which lowers the voltage across the filament and results in a very inefficient lamp with dramatically lower light output.
A very large number of negative temperature coefficient thermistor materials have been developed. Most thermistors (including undoped polycrystalline vanadium dioxide) have dramatically higher resistances at -20.degree.C (which an outdoor lamp might be exposed to on a cold day) than at +25.degree.C (a typical indoor ambient temperature). Thus a current limiter designed for indoor operation would take an extremely long time if used outdoor and thus be impractical for outdoor operation.
Futaki and Aoki (Japanese Journal of Applied Physics, Volume 8, No. 8, pages 1008-13, August 1969) have reported a number of various doping elements for vanadium oxide semiconductors. These elements include MoO.sub.3 together with P.sub.2 O.sub.5 in relatively large amounts (atom percents of 0.1 or more of MoO.sub.3 and 1.0 of P.sub.2 O.sub.5 /2). While such relatively heavy doping does flatten the low temperature portion of the resistivity curve, such doping shifts the transition temperature down to about 55.degree.-60.degree.C which provides operating problems in high ambient temperatures and also reduces the cold-to-hot resistance ratio to a value of less than 20 and thus is completely unusable in a practical limiter configuration.